Silently catching numexpr errors and changing vm can be confusing

[FrancescAlted]

There is an example of that in: pydata/numexpr#150 (comment)

Perhaps a better solution would be to raise an error in case the "numexpr" vm cannot handle an expression and inform the user to use other virtual machine ("python" or "dask"). Opinions?

[thequackdaddy]

What about a warning instead? An outright error seems extreme to me.

[FrancescAlted]

Yes, I was also thinking that a warning would perhaps be the right thing to do. Implemented that in 382c969.